The present invention relates to vessels for storing hazardous, obnoxious or valuable or sensitive fluid materials. It more particularly relates to vessels for safely transporting and handling such fluid materials.
Society today requires that numerous chemical materials be handled, many of which have hazardous or obnoxious properties. These materials include for example acids, alkalies, chlorine, ammonia, liquefied petroleum gases, hydrogen sulfide, hydrogen cyanide, sulfur dioxide, mercaptans, fuels, pesticides, radioactive materials and industrial wastes. To ensure that these hazardous, obnoxious, or valuable or sensitive materials do not escape into the environment during their processing, storage and transportation, they are contained in strong vessels or piping systems. These vessels must not only provide satisfactory access to the contained materials, but must completely and safely contain them at all times when the escape thereof to the outside environment is undesirable or unsafe. In some cases, it is even desirable to protect the stored materials itself from the environment.
The unintentional escape of such substances from their containers can have disastrous consequences, including the loss of life, damage to health or property, public inconvenience and even the evacuation of public areas. Accordingly, there is a strong need to provide safer containment systems. Valves with or without mechanical actuators to operate them are used to access the materials stored in the sealed vessels. The containment vessels are typically reliably built. It is the valves thereof and the attachment of the valves which are the weak points in the containment system and thereby reduce the reliability and usefulness of the entire containment system.
In some instances, relatively large leaks or seepages from valves are tolerated by users and by society depending upon the particular location and the state, pressure and properties of the stored materials whether hazardous or non-hazardous. However, in the case of extremely toxic, reactive, obnoxious, valuable and sensitive materials even small failures of containment or seepages can be so objectionable as to discourage or even preclude the handling, transportation or storage of these materials. This problem is growing due to the public's increasing anxiety over the handling of chemical and radioactive materials by both industry and government. Materials which exist in normal conditions as high pressure or liquefied gases are particularly troublesome, especially if the materials have a foul odor or corrosive properties. Seepages may not even approach hazardous levels before the users of the materials are exposed to adverse publicity, litigation and extremely stringent and costly regulations. When valve systems used with hazardous, obnoxious or valuable materials fail, the release of the materials can have potentially lethal and costly consequences. This failure can result form highway accidents, fires, explosions, earthquakes, storms, misuse, abuse and vandalism.
Nuisance leakages from transportation vessels are characterized by small fugitive emissions from vessels. Such leakages may or may not be inherently hazardous, but when detected they are almost always regarded by the public with great fear and alarm. This can cause great embarrassment and expense to shippers of hazardous materials who often must fly in repair crews to repair or deal with such leaks. Negative publicity and further costly regulation of the shipper's activities may result. The spector of litigation, whether for real or imagined damages, is always present when there has been a leakage.
Nuisance leakages almost always arise from defects or failures of vessel closures to perfect seals. They only very rarely result from plate or welding defects in the vessel itself. Flanged or gasket closures are the most reliable, followed closely in reliability by properly sealed threaded plugs or caps. Both are readily tested for leakage before shipment, and when this is done, seldom seep en route. Both are relatively strong and resist impacts and other abuse. Valves are the principal leaking culprits, since they are relatively complex devices with moving, rubbing and wearing parts and are generally equipped with friction seals on their packing glands. They typically protrude considerably from the vessel and are therefore vulnerable to damage. On the other hand, a vessel without valves is not very useful since one cannot easily gain access to its contents, if they are under pressure.
To protect the protruding vulnerable valves on hazardous material transportation vessels, rigid steel protective domes are typically erected or constructed around the valves. Sometimes excess flow valves are installed inside the vessel. Sometimes a portion of the valve body is installed inside or partly inside the vessel and the activating portion is left outside where it is ready to transfer impact damage to the valve itself. Conventional manway entrances to tank cars and trailers consist of simple hatches or flange systems on protruding, vulnerable nozzles, just as on conventional stationary vessels.
When valve leaks occur in transportation vessels in transit, crews are dispatched generally by airline to attempt "hot" repairs to the leaking pressurized valves. If these repairs fail, a few hazardous materials vessels are equipped to receive "valve safety kits" which are clumsy devices designed to fit over the entire valve and seal (more or less) to the vessel exterior. Since this exterior is often dirty, damaged, corroded or otherwise rough, it is difficult and sometimes impossible to make a good bubble-tight seal to the vessel with these kits. These kits are also difficult to transport, especially on commercial airliners, and are heavy and cumbersome to use.
Catastrophic failure of transportation vessels and especially those carrying pressurized gases or liquids often results when their valves or nozzles are impacted. Conventional valve and fitting designs mounted at least partially outside of the vessel are vulnerable to impact, damage or being shorn off when their vessel is in a wreck. Valves, nozzles and manways of such vessels protruding outwardly from the vessel are vulnerable to flying debris, other vehicles or tank cars, railroad irons, bridge abutments, tunnel walls or overpass supports.
Relief valves of conventional vessels are especially vulnerable since they cannot be protected from impact damage or shearing by ordinary excess flow valves within the vessel. The only protection afforded relief valves is that provided by the protruding valve enclosure or tank dome or similar external structure. If they are equipped with excess flow valves, they cannot then function as relief valves. Further, the fitting domes of conventional vessels protruding from the tanks do not adequately protect the fittings therein or the dome (or manway nozzle) itself.
Conventional containment vessel designs with valves and manhole nozzles do not approach the potential reliability levels of the simple cylindrical or spherical containment vessels shapes to which they are often attached due to the structural compromises made to place the nozzles or valves on the exterior of these vessels. The external nozzles systems thereby are weak points in the containment vessel and compromise and reduce the reliability and usefulness of the entire vessel containment system.
Protruding nozzles are relatively weak structural points subject to shear failure in the vent on an impact. Their failure can result in the catastrophic release of the contents of the vessel even though the vessel itself remains essentially intact. On the other hand, a vessel without means of access is essentially useless, and thus valves are needed to add, withdraw and monitor the contents of the vessel. Also, personnel access is often necessary to properly maintain the interior of larger vessels. For tank cars, highway trailers, cylinders and process vessels, these utilitarian purposes have resulted in designs which compromise the inherent strength and impact resistance of the vessels themselves. Thus, today's vessels are often subject to unnecessary breaching when impacted.
Prior art process and transportation vessels have been designed 1) to maximize the ratio of vessel volume to vessel wall volume, 2) to maximize the ratio of vessel pressure rating to vessel wall thickness, 3) to maximize the use of simply formed component shapes such as cylinders and flats and to a lesser extent spheres, hemispheres and ellipsoids, and 4) to maximize the use of standard valves and other piping appliances. Designers have tended to believe that the maximum forces to which the vessel will be exposed are the ordinary forces of static design internal pressure, normal transportation forces, gravitational forces, wind pressure, ambient temperature gradients and the like.
Limitations on the vessels' diameter or width imposed by the necessities of travel along railways and roadways and the ease of construction have resulted in the general use of cylindrical vessels with hemispheric or hemi-ellipsoid heads. These shapes tend to address the first three goals listed above very well. However, in addressing the fourth goal designers have merely added needed valves and appliances in the most obvious manner--by breaching the smooth exterior of the vessels at convenient locations and installing one or more nozzles projecting to the outside. These nozzles normally terminate in a standard flange, to which a valve or other flange can be mated, thereby effectively sealing the vessel. Where the breach is sufficiently large to compromise the integrity of the vessel at its normal thickness, reinforcing bosses are welded to the vessel wall, usually on the exterior. These projecting nozzles typically extend two to twelve inches from the vessel wall surface to provide room for bolting operations and vessel insulation where needed. This method of adding nozzles seriously harms the integrity of the vessel, however, particularly in its ability to withstand random impacts during wrecks, derailments, topplings, explosions and the like. These nozzles themselves, as discontinuities projecting from the surface of their vessel, provide convenient purchase points for impacting objects and are subject to destructive shearing. Furthermore, the resulting location of the attached appliances, such as relief and other valves, indicators and manhole covers, makes these devices vulnerable to impact and fire damage in the event of an accident.
These problems have been partially addressed in the past by one or more of the following design changes:
1. installing internal excess flow valves on certain nozzles to prevent the loss of contents in the event of total shearing off of the exterior nozzle;
2. machining intentional weak points or break-off points in the nozzles to prevent the transmission of impact stresses from the nozzle piping to the vessel wall;
3. repositioning nozzles on the vessel from locations particularly vulnerable to impact to less vulnerable areas;
4. using supplemental external reinforcement for some of the nozzles;
5. constructing external guards around and over external nozzles and fittings;
6. using specially designed external valves better able to withstand impacts and fire; and
7. using specially designed valves mounted partly internally to reduce exposure to impacts and fire.
All seven of these remedies, while somewhat effective, are only band-air attempts to remedy flaws inherent in the expediency of attaching unprotected external nozzles to the pressure vessels in the first place. Their drawbacks are discussed below.
First, the installation of internal excess flow valves is only practical on nozzles attached to external valves and not on relief valve nozzles, manholes and the like. Further such devices are only directed to the escape of material at rates in excess of the rated flow of the device. Smaller leaks are therefore unimpeded by excess flow valves, yet smaller leaks resulting from fire or less than total failure of the external valve or nozzle are the most common in accidents.
Second, the purposeful machining of weak points or breakpoints is only useful if there is some other device upstream, such as an excess flow valve, which stops the massive flow resulting when the breakpoint is shorn off. Such devices cannot be used on relief valves and manway nozzles.
Third, at the insistence of regulatory bodies, such as the U.S. Department of Transportation (DOT), outlets are generally prohibited in such obviously vulnerable locations on transportation vessels as the bottoms and ends of tank cars carrying flammable gases and liquids. Therefore, the nozzles are moved to the top of the vessel which is an area less likely to suffer impacts. Unfortunately, three problems are thereby created. (1) The unloading of liquefied compressed gases is complicated since the pumping of the liquid requires the lifting of a liquid at its boiling point to the suction of the pump which results in cavitation. This requires cavitation tolerant or high maintenance pumps or pressure unloading which suffers from its own hazards. (2) Even non-boiling liquids must be pressure unloaded with the attendant risk of introducing excessively high pressures or inappropriate (potentially reactive) substances into the vessel during unloading operations. (3) By moving the remaining unloading position to the top of the transportation vessel, the workers involved in unloading and/or loading of these cars must necessarily work at the highest level on the vessel in a stopped position. This can result in worker discomfort, the likelihood of falling accidents, the aggravation of back injuries and working in a location where escape from accidental leakages is most difficult.
Fourth, the principal drawbacks of reinforcements are that the area of possible purchase by an impacting force is increased in proportion to the size of the reinforcement and that the reinforcement adds weight to the vessel. This extra weight ultimately reduces the vessel's effective ability to contain materials, especially in transportation uses where weight is critical.
Fifth, guards are commonly installed around small nozzles and normally take the form of removable heavy caps, as in compressed gas cylinders, or "dome" arrangements as in tank cars and some tank trucks. The domes typically comprise steel cylinders bolted to the vessel, equipped with heavy covers and containing within them the small vessel values, monitor ports and relief valves. Again, these devices must be massive if they are to deflect a major impact, and this additional weight is a major disadvantage in transportation vessels. These guards also form a discontinuity in the smoothly curved surface of the vessel thereby increasing the likelihood that the dome and its contents will be shorn off following a major impact. As a variation of the fifth solution, guards have been used in some earlier experimental transportation vessels wherein the dome was "inverted" and placed in a recess more or less within the smoothly curved envelope of the vessel.
Listed below are patents which may be relevant to the present invention. The following patents related to recessed wells in fluid vessels: U.S. Pat. Nos. 2,006,924 (Kizer), 2,048,454 (Kizer), 1,759,734 (Davenport), 2,747,602 (Trobridge), 1,627,807 (Roussie), 1,933,233 (Wakefield), 2,067,993 (Thwaits), 2,723,862 (Dalglish), 2,858,136 (Rind), 3,884,255 (Merkle), 3,889,701 (Mueller), 3,081,104 (Schmiermann), and 2,096,444 (Arvintz). The following patents related to diametric and/or pressurized wells in fluid vessels: U.S. Pat. Nos. 3,341,215 (Spector), 2,548,190 (Arpin, Jr.), 1,542,116 (Welcker), 1,442,525 (Howard), 715,355 (Dees), 113,153 (Fisher), 1,053,344 (Asbury), 1,699,527 (Folmsbee), 2,675,794 (Armstrong), 3,157,147 (Ludwig), 3,658,080 (Mitchell), 3,883,046 (Thompson et al), and 4,085,865 (Thompson et al.). The following patents related to rupture discs: U.S. Pat. Nos. 3,310,197 (Folmsbee et al.), 3,845,878 (Carlson), 4,183,370 (Adler), 4,553,559 (Short, III), 4,245,749 (Graves), 2,092,925 (Lithgow), and 3,109,555 (Samans). The following patents relate to control valves: U.S. Pat. Nos. 1,544,024 (Moeller et al.), 1,897,164 (Endacott), 2,423,879 (De Frees), 3,187,766 (Black), 3,310,070 (Black), 3,764,036 (Dale et al.), and 4,009,862 (De Frees). An internal valve assembly is shown in U.S. Pat. No. 4,872,640 (Schwartz). Related U.S. applications are Ser. Nos. 07/595,477, filed Oct. 10, 1990, and 07/594,171, filed Oct. 9, 1990. The entire contents of each of these patents and applications and any other patents, publications or applications mentioned anywhere in this disclosure are hereby incorporated by reference in their entireties.